Here we give the details about the used ODE solvers and their used implementations. We implemented the explicit non-adaptive RK methods as user-defined functions in Stan. The RK45 and BDF solvers are built into Stan, and we describe how Stan uses the Boost Odeint [1]} and SUNDIALS [2]} libraries to obtain both the ODE solutions and their sensitivities. The goal is to focus on how the specified tolerances and other factors affect the approximate solution and sensitivities. We used Stan version 2.28, which depends on Boost version 1.75.0 and SUNDIALS version 5.7.0.
